linux-kernel - Re: [PATCH 0/5 RFC] Add an interface to discover relationships between namespaces

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Message-ID: <20160723215802.GO24913@odin.tremily.us>
Date:	Sat, 23 Jul 2016 14:58:02 -0700
From:	"W. Trevor King" <wking@...mily.us>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
Cc:	Andrey Vagin <avagin@...nvz.org>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	linux-api@...r.kernel.org, containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>, criu@...nvz.org,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	linux-fsdevel@...r.kernel.org,
	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
Subject: Re: [PATCH 0/5 RFC] Add an interface to discover relationships
 between namespaces

On Sat, Jul 23, 2016 at 02:38:56PM -0700, James Bottomley wrote:
> On Sat, 2016-07-23 at 14:14 -0700, W. Trevor King wrote:
> > namespaces(7) and clone(2) both have:
> > 
> >   When a network namespace is freed (i.e., when the last process
> >   in the namespace terminates), its physical network devices are
> >   moved back to the initial network namespace (not to the parent
> >   of the process).
> > 
> > So the initial network namespace (the head of net_namespace_list?)
> > is special [1].  To understand how physical network devices will
> > be handled, it seems like we want to treat network devices as a
> > depth-1 tree, with all non-initial net namespaces as children of
> > the initial net namespace.  Can we extend this series'
> > NS_GET_PARENT to return:
> > 
> > * EPERM for an unprivileged caller (like this series currently does
> >   for PID namespaces),
> > * ENOENT when called on net_namespace_list, and
> > * net_namespace_list when called on any other net namespace.
> 
> What's the practical application of this?  independent net
> namespaces are managed by the ip netns command.  It pins them by a
> bind mount in a flat fashion; if we make them hierarchical the tool
> would probably need updating to reflect this, so we're going to need
> a reason to give the network people.  Just having the interfaces not
> go back to root when you do an ip netns delete doesn't seem very
> compelling.

I'm not suggesting we add support for deeper nesting, I'm suggesting
we use NS_GET_PARENT to allow sufficiently privileged users to
determine if a given net namespace is the initial net namespace.  You
could do this already with something like:

1. Create a new net namespace.
2. Add a physical network device to that namespace.
3. Delete that namespace.
4. See if the physical network device shows up in your
   initial-net-namespace candidate.
5. Delete the physical network device (hopefully it ended up somewhere
   you can find it ;).

But using an NS_GET_PARENT call seems much safer and easier.

Cheers,
Trevor

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